Hopefully you have your XP installation disk. Put it in the cd drive and hopefully the machine will be able to boot to the CD, no drivers are required to do this. If not, you may have to change your system bios setting to allow it to boot to the CD first. When the setup program loads, tell it you want to Install , not repair. The first thing the installer will do is look for an existing installation when it finds your existing one, it will ask if you want to repair or do a new install. At THIS point tell it you want to Repair. Normally this works ok and you will end up with a functioning system with all your applications still installed. If it comes up working, do a complete scan for viruses and Malware. You should use all the tools to do this that are available because no one does it all. Read the Sticky posts at the top of the Security forum for all the information you need on how to do this. You will need to reinstall all service packs and updates again when you're done.